This invention relates to silver halide photographic emulsions. More particularly, it relates to silver halide photographic emulsions which are improved in sensitivity, gradation, fog, density, and time-depending stability.
There have heretofore been known various methods for increasing the sensitivity of silver halide photographic emulsions. As examples, mention may be made of generally known noble metal sensitization which employs compounds of noble metals such as gold, platinum, and iridium; sulfur sensitization employing sulfur compounds such as thiourea, sodium thiosulfate, and the like, reduction sensitization using stannous salts, polyamines, and hydrazine derivatives; and development acceleration by use of quaternary ammonium salts and polyalkylene glycols. It is also well known that more sensitive silver halide photographic emulsions are obtained by applying the above methods of sensitization to the silver halide emulsions comprising silver bromide as major constituent, especially silver bromide of increased grain size. Such an emulsion, however, has a disadvantage of soft tone, being low in maximum density. A common technique used in increasing the contrast, such as, for example, the well known addition of a rhodium salt accompanies desensitization and, in addition, is not sufficiently effective for the silver halide emulsion containing silver bromide as major constituent. Such desensitization cannot be compensated by chemical sensitization, because intensive chemical sensitization causes fogging, which imposes a limit upon the attainable sensitivity. One of the methods to attain a high maximum density with unit amount of silver halide is to use a fine grain emulsion. This method, however, accompanies also a decrease in sensitivity.
Spectral sensitization (optical sensitization) is a well-known technique to extend the sensitive region of silver halide to a longer wavelength region of the spectrum. For this purpose, there are known a great variety of sensitizing dyes such as cyanine dyes and merocyanine dyes. Although various performance characteristics are required for the sensitizing dyes, general requirements are a sufficiently high sensitivity in the intended sensitive region and the complete decoloration of the dye after completion of the photographic processing. As the dyes which meet such requirements, there are known cyan dyes having a water-soluble sulfoalkyl group at position 3 (and 3'), that is, cyanine dyes of the betaine type (and anionic type). However, because of its comparatively low adsorbability, such a cyanine dye tends to be hindered from adsorption onto the silver halide grains by the presence of an antifoggant, particularly an organic compound having a mercapto group, which is used to alleviate the fogging caused by the chemical sensitization. The sensitivity (spectral sensitivity) imparted by the sensitizing dye becomes gradually decreased with time, especially with time elapsed before coating on a support, and, in an extreme case, becomes substantially null.